Debugger: Cleanups.

VariablesView:
- Remove unused member variable.
- Add optional presentation name field to ModelNode, which allows
  overriding the name of its underlying ValueNode for display purposes.
  Used to ensure that a top-level node for an expression always shows
  the expression string itself, even when the returned result is a value
  node with a different name.
BCLV: Adjust arrow key behavior.

- Left/right arrow can now be used to collapse/expand the focus
  row. Holding shift reverts to the previous behavior of scrolling
  the view.